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Technical Abstract: Nitrogen (N) enters and leaves a dairy production system through many pathways and in many forms: undergoing numerous transformations as it passes from feed to animal to milk or manure and back again. Due to the complexity of the dairy system, estimates of N flows and losses require the use of models to ensure all pathways are accounted for and to assess how changes to one part of the system affect another. The Integrated Farm Systems Model (IFSM) was used to simulate a large dairy farm in the Western, Midwestern, and Eastern regions of the US. Simulated farms had the same number of animals (2000 lactating Holsteins; 800 heifers > 1 yr; 800 heifers < 1 yr) with varying stocking density based on regional practices (5, 2, and 1.7 head/ha respectively). A target milk production of 11,000 l/animal/yr was set for all simulations. Four manure management scenarios were simulated for each region: A) uncovered basin storage with broadcast application; B) covered basin storage with broadcast application; C) uncovered basin storage with manure injection; D) and covered basin storage with manure injection. Model outputs were averaged over 25 weather years. Whole farm N efficiency, estimated as the proportion of N imports exported in milk and meat was 0.29, 0.35, and 0.37 for Western, Midwestern and Eastern farms under scenario A. The location and form of major N losses and response to manure management scenarios varied with region (Table). These results highlight that stemming N loss from one location often increases loss in another. Holistic assessment of N management is required to find solutions that increase farm level N efficiency rather than simply diverting N losses.
Table: Size of nitrogen loss and percent change with management changes